1. Field of the Invention
The present invention relates to long stroke well pumping units, and more particularly to pumping units of the counterbalanced type.
2. Description of the Prior Art
As the shallow deposits of crude oil are depleted wells at a greater depth appear with increased frequency. Characteristically the pumping of an oil well entails reciprocal articulation of a string of sucker rods and as the well depth increases the elastic component of the string often masks the motion of the downhole pump suspended on the end of the string. As a consequence pumping of deep wells is best achieved by long, low frequency, strokes, and at a stroke rate necessarily below the fundamental resonance of the rod string.
Additionally, low frequency pumping of oil wells has been recognized in the past for the conservative aspects thereof in mechanical loading, aspects which both reduce any fatigue cycle rate and more importantly, allow for more precise balancing and thus a more even power profile. (Typically, a well pump expends power to lift the fluid head in the course of each stroke. Superposed on these power demands are the power losses associated with stroke reversal, excitation of rod string modes, and other mechanical components which are not entailed in lifting the fluid. These parasitic losses diminish inversely with the length of the stroke and directly with the reduction in its frequency.) Accordingly, long stroke pumping of deep formations has had wide acceptance, acceptance which increases with the number of deep wells presently pumped.
In the past, long stroke pumps have been variously implemented. One technique utilizes cable and drum arrangements like those taught in U.S. Pat. No. 4,062,640 to Gault, or in U.S. Pat. Nos. 3,285,081 and 3,528,305 to Kuhns et al. Alternatively, the use of chain and sprocket is taught in U.S. Pat. Nos. 2,520,187 to Wilshusen et al; 1,637,078 to Hill; 1,927,831 to Hild, and 4,179,947 and 4,197,766 to James. All of the foregoing, while suitable for their purposes, operate at a counterbalance point which substantially balances out the well column weight. The prime mover, therefore, is loaded in the course of reversal of each stroke by the combined inertia of the rod string and the counterbalance mass. This dynamic component requires complex mechanisms to control the large component of wear induced by parasitic loads during reversal. The control over this dynamic component is thus a major effort in the prior art and it is this dynamic component that is conveniently resolved herein.